Yea I'm currently sitting on 94% for items and 100% for the other 3 conditions and I've got a pretty strong feeling that you have to get each item for each character. This may even include the final recipes from the mirage arena, as I need 3 physical go-into-your-inventory items to complete terra and I don't thinks they're going to be enough to push me to 100%.
I'll do an experiment though, I haven't gotten all of the Ice creams in my aqua file yet and if I get only her exclusive ones without getting the common ones and I still get to 100% then we can be sure that its not a necessity to have the items as all 3 characters. I can be certain that all 3 Journals don't have to be complete for combat to hit 100%
Also, and this may have been a coincidence, but I think I may have unlocked 100% in combat by beating MF the first time. I remember checking at a time before beating him when I was on 99% and then checking after his defeat and seeing it at 100% and I can't remember what I did in between. Although I may have gone to do the final fight at the mirage arena in that time. Can anyone else verify whether or not its neccesary? I'm pretty sure that it wouldn't be